Finally, thank the heavens, summer is here! Rejoice, as all your worries are cast away… almost.

As per usual, summer comes with three caveats: Sunburn, Crowded beaches, And glorious blue skies.

While we may know nothing about the fallibility of your skin, or why beaches are so popular during the summer months, we can certainly elucidate why that lovely undulating presence radiates above you in the summer months.

So then, why is the sky blue?

There are three things we have to consider when we think about the colour of the sky:

1. Sunlight
   
2. The Atmosphere
   
3. Rayleigh Scattering
   

The Sun

The Sun’s light is not emitted in the unified singular rays we see with our naked eye, instead consisting of many different colours. Using a prism, we can more clearly split sunlight into its components, made up of waves travelling at different frequencies, which are thereby represented by changes in colour and energy.

This matters because it demonstrates that all of the colour we see around us is present in white sunlight. Making the colour of the sky a by-product of the sun.

“So - if sunlight is colouring the sky, why does it change from white to blue?”

Excellent question!

The Atmosphere

Our atmosphere shares some responsibility for the colour of our sky. This is due to its chemical composition, which contains lots of nitrogen, oxygen and dust, all of which interact with light travelling through them. Earth’s atmosphere lends itself to the blue colouration, due to an effect we’ll explain shortly.

Interestingly, the atmospheres of different planets influence the colour of their skies. You probably wouldn’t fancy a holiday to Mars, due to an average surface temperature of -60oC… but if you found yourself surreptitiously placed there, the high dust and CO2 content of the atmosphere would leave you a midday sky of reddish amber, or a sunset of blueish grey. Conversely, the Moon would provide no sky to look at whatsoever, due to its almost non-existent atmosphere.

“So, what’s happening between the light and the atmosphere to create the colours in the sky?” I hear you ask.

Another excellent question!

Rayleigh Scattering

After a great deal of skyward pondering, in 1871 Lord Rayleigh argued that waves of light colliding with tiny atmospheric matter must project it in all directions, resulting in the different colours we see in the sky.

Today, we call this Rayleigh Scattering, and it describes the way that light of shorter wavelengths (such as blue and cyan) is scattered more than light of longer wavelengths (like red and orange). This also means that the angle of the sun impacts the colour of the sky.

At the peak of the midday sun, the scattering of sunlight illuminates the sky with a beautiful bright blue colour. However, in the early evening as the sun is sinking, the light travels through more of the atmosphere, so the blue waves are scattered away entirely, meaning that the light left behind is red and orange.

A Closing Thought

Hopefully we can now understand the basis of why the sky is coloured so beautifully at this time of year. To avoid feeling too proud of ourselves, we leave you with a final point of perplexion.

If the sky is blue in the middle of the day, why do clouds appear white?

Sources:

1. Britannica.
2. Space Place.
3. UNI VIE.
4. RMG.